<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      .one {
        border: 5px double black;
        width: 500px;
        height: 500px;
        padding: 10px;
        margin: 10px;
      }
      .two {
        width: 200px;
        height: 200px;
        background-color: aqua;
        display: flex;
      }
      .two > div {
        /* flex会导致内部盒模型宽度不在默认100%需要自己手动去设置 */
        /* width: 100%; */
        background-color: bisque;
      }
    </style>
  </head>
  <body>
    <div class="one"></div>
    <div class="two">
      <div>12</div>
      <!-- <div>23</div> -->
    </div>
  </body>
</html>
